Meta revealed a number of new features coming to Facebook Messenger, and if you haven't noticed them yet, it's most likely a staged release, so give it a few of days. The most significant change is that end-to-end encryption is now enabled by default.
All of your private communications are now encrypted, and only you and your receiver may view them. That's a significant step forward, given most competing messengers, not just privacy-focused ones, currently provide end-to-end encryption.
In addition to encryption, you may now modify messages (a long-awaited feature), send vanishing messages, conceal your read receipt so people don't know when you've seen their message, and voice chatting has been improved. For example, you can listen at 1.5x or 2x speed and resume listening to communications where you left off the previous time. There is a catch, though: you may only modify messages that have been sent within 15 minutes.
Photo and video quality have been improved, and Meta is currently testing HD photographs and videos with a select sample of users before rolling out the updates globally.